The ability to image microstructures, such as the micro-vascular network in the skin, the GI tract, or the brain cortex, and to monitor physiological functions of tissue is invaluable. One of the promising technologies for accomplishing this objective is photoacoustic microscopy. Current high-resolution optical imaging techniques, such as optical coherence tomography, can image up to approximately one transport mean free path (about 1 mm) into biological tissue. However, these techniques are insensitive to optical absorption that is related to important biochemical information. Other well-known techniques, such as confocal microscopy and multi-photon microscopy, often involve the introduction of exogenous dyes, which, with a few notable exceptions, have relatively high toxicity. In addition, acoustic microscopic imaging and spectroscopy systems are sensitive to acoustic impedance variations only, which have low contrast for early-stage cancer and provide little functional information about biological tissue except flow. In contrast, photoacoustic wave magnitude is, within certain bounds, linearly proportional to the optical absorption contrast; thus photoacoustic spectral measurement can be performed to gain functional (physiological) information such as the local blood oxygenation level.